Frequency shift keying



Dec. 28, 1948.`

original Filed Feb. 11, 1944 G. L. USSELMAN FREQUENCY SHIFT KEYING 2 Sheets-Sheet 1 Dec. 28, 1948. G. l.. UssELMAN 2,457,288

` FREQUENCY SHIIFT KEYING I I Original Filed Feb.v 1l, 1944 v 2 Sheets-Sheet 2* www@ "fill-I .ws/va Jaa/ref IN V EN TOR.

'wyfllffcb/m Patented Dec. 28, 1948 2,457,288 FREQUENCY SHIFT KEYING George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application February 11, 1944, Serial No. 521,907. Divided and this application February 7, 1946, Serial No. 646,069

' This invention concerns frequency modulation and in particular an improved method and means of communication by frequency shift or spaced wave keying. This application is a division of 7 claims. (cl. 25o-s) I cies for the mark and space characters of the prior art, but none of the disadvantages.

In describing my invention'in detail, reference will be made to the attached drawings, wherein my United States Application Serial #521,90'7, V5. Fig. 1 is a block diagram illustrating schematiu filed February l1, 1944. By this method of comcally the general arrangement of a spaced wave munication carrier wave energy keyed from a keying system arranged in accordance with my first to a second frequency and Vice versa is proinvention. duced. The first frequency may represent mark- Fig. 2 illustrates diagrammatically the essential ing condition while the second frequency may 1Q ielementlof a'lilnractia Splced wavelkeyinf sysirepresent spacing condition. The carrier wave em SUC as 1 11S Ia e SC ema 10a y 1n igeeeeey ee keyed mee ee eeeeeeeee ee feeeefjee tr ettlv ytererhrestrnd1i from a single source the frequency o W ic is S D S a 2 y alternately keyed or may be considered as sepay Xed IS and Second fIeClU-BHCGS and t0 i r1 turn rete carriers alternately translated, the aiterna- 10 alternately Operate through an electronic and tion being in accordance with signals such as, for fnenamcll SWitehmg arrangement to synchroexample, telegraphy, facsimile, code orhtelevisin. lgeallrgli?liielpfucirrlietlilclrd At the receiver both frequencies, t at is, e mark and Space or first, and second frequencies tor is of the regenerative type. Athird stabilized are used in reproducing the signal, thus giving OSCHaOI perating at the carrier frequency mary the benets of positive action on mark and space, also entrain the th1rd oscillator when the same 1s the benet of frequency diversity, em not entralned by one of the two rst mentioned The use of two frequency sources for frequency 050133130115- n shift keying as described above has been disclosed 3 1S nmodlcnton 0f the 91T nngnlnenl? 0f in the art as has been the use of the mark and t1g. wherein the lower half of the electric switch space frequencies at the receiver. However, ug? V nud-the nrczn arelmlttedf method and means known heretofore for accom- Fglgalllsda m0 1 @a lon 0f e arrangement 0 plishing the keying of the twocarrier waves re 2 suited in amplitude keying thereof with au its Flg. lllufrltes a nodlfigdtonf eertroued attendant disadvantages. In the prior systems, gentera 01' Whlc may e su Sl u e 0T t e gen even if amplitude keying were not present or did eragifr; neo prgrFigulrevo er Stal Osciutors not accompany the frequency shift keying the m and 2n ar Wrovd'l ne f ryth ark fre change from the nrst to the second frequency is d l p ,di f 0 th 0 e n so abrupt that there reslts a wide dispersion olf glrlnelglesfol tesgrtwoecgg secsthe side frequencies. T is extends he channe y causes overlapping of 'lide bands and produces slgvedlhscr rarllssl; keying clicks, etc., in t e circuits. f

An object of the present invention is improved gafrargsvlgrremllfg Siigl r'gr frequency .Shift keying An addtiona1.ob]ectof 40 source 3l] and as is known thxis signal comprises uihmvte m10?. ,s mpm/eld reqully hlftdkeymg electrical energy representing mark and electrical W1 gut ampi; ut etaokuaj lonfo ih es; Nwe energy representing space, so utilized that rst so t a m e ec e eymg ro'm e rs re' one crystal oscillator output then the other is qflency to the Second frequency 1S smoothly car' switched on or through to deliver crystal conrled out therebyreducmg the 51de band frequency 45 trolled excitation to a controlled or synchronized Spectrum. reducmg keylng s nocks etcoscillator in 5U. The oscillator in 5U may, when In a'ttalnlng the above ObJeCtS I use tWO 00nuncontrolled, operate at a frequency intermediate Stam frequency Sources but a mthod and means mark and space, but is of a nature such that it is iS included fOr the frequen'y Shift keying that smoothly pulled into synchronism by whichever hds the amplitude keying modulation t0 a minv5() source l0 or 2l] is switched through at 40 under imum and also prevents sudden frequency the control of signals at 30. The mark and space changes. My method and means thereby prefrequency energy is then amplified and/or frevents unnecessary widening of the frequency quency multiplied in 60 and utilized say,'for exband. Thus the present frequency shift system ample, radiated. retains the advantage of accurate xedfrequen- The oscillator 5B is of atype that while it oscillates continuously it is easily controlled and synchronized by either of the crystal controlled oscillators switched on by unit 40. Oscillator 50 is adjusted so that during the intervals of switching it continues to oscillate on a mean frequency with substantially'no changefin amplitude: The output of controlled oscillator uniti/Stills. next: amplined and/or multiplied in frequency in unit 60 and then radiated by the antenna. The oscilla. tor in 5t is arranged so that it maintains alsubstantially constant frequency output by selffeexl citation but readily changes, its frequency tasln chronize with that of the. crystat excitation switched on by the signal from 30 through unit 40,

It can be seen that the oscillator' 5U provides a buffer or in eiect a fly-Wheel between' thee-two*y crystal oscillators l and 2U, andthe transrrritterx amplifiers and/or frequency multipliers in 631 The action is two-fold. The oscillations con-- tinue at the mean frequency and at substantially constant amplitude `during-the time when neither crystal isf in control, thusI the receivenhas acarrierto' operate ont continuously..v Thesecond actioni is tha-t4 several:l cycles ofi oscillationl in: unit 50i occur. during,y the" time: off .frefpiencyl change from' mark. to space; andA viceif versa; Thus the lreyin'giis smeet-hf sincethetiinatrequency change is not too sudden and side frequencies are not spread soffar. Notefthat. actually` in` my: system tlzu'ee'l frequencies areinvolved; these frequencies beingl mark, spaceandlno signal- Ins Fig'. 2, V4 is an4 oscillator of:4 a modified Hartley' type having its anodey coupled to its screen.' grid'by.- atuned: LC circuit including condenser Gill' and nductairce Isl' with a point on the.l inductance LlC connectedi toY the cathode: by coupling: oonden-seirO'.- Oscillationsiare generated inv the: tube." Vil and. cineuitf` byf virtue or. the factI that/enough! electrons. are drawn past the' control: grid. at. all' times to; supportv oscillae tions.. The condenserN'f is ai feedbackr or regeneratiom controla condenser' anni 25xk is` a resistance in shunt to this condenser by meansofwhich positivevbias is supplied tothezscreen gridl oft tube Vl. Thef anode' derives? direct current potential from the main sourccodirect; current;

'Ehe-control' gridiof: tubeV-t is connected tot the K cathode by potentiometer resistance 27| and also byaf'potentiometer resistance 2.3;. apoint' of which is coupled:- by" resistance. 3.!! to: the positive: termi;- nal of the direct curr-env source:A The purpose of potentiometer* Iltis to adjust t'hczfbias-.as' desired ont the control! grid of: this tube: The potentiometer 21 has. two taps: thereon and providesf a means; for applying.;` the synchronizing; or con trolling voltage tcthegenerator'of, tubeVlf. Any excitation applied' to: the control gl'ld or any tube element ot the oscillator tube? Vlr willu havea tendency' to control-z the A frequency ot' oscillations generated inY tube VL. thereby: sync'lfrronizingl the same/With the-controlling potentials-, providedthe natural oscillatingv frequency ofv the tube Vfh is not tooI farr different. from thev excitation or controlf. frequency. Al controlled, oscillator` ot this generali, type has-boem shown; and describedI in detail irrfEign` 3: of Usseiman. Patent No. 2,425,165 dated Alugust 5, L95?. Moreover, the oscillator while stabilizedas'` to a-v particular frequency is readily swung under control, of. the synchronizing oscillations: about. thisl particular frequency, to wllichi` it iss. tuned, by thes-ignals operatingon the switch.

Irn my'i-mlprovedlsystemaai tap on the potentiometer resistor 21 is coupled to the' anodes of the .tubes Viaud V3i, by way of direct-,current blocking and alternating current coupling condenser O5.

Tubes V2 and V3 work in the amplifier and switching circuit which may be included in the switching unit 40 in Fig. l. The anodes of tubes V2 and; V3:t arel connected by a radio choke L3 to. the positive terminal of the main supply source. In this modication the generator at Vi isgcontrolled by oscillations from ill or 2d through the switching means including tubes V2 and V3. Note that' the positive terminal of the direct current source is connected to the screening electrodeslof, tubesV and V3 by the relay contacts 2fl.-,.2'.2l.and. 23. This connection includes a potential adjusting potentiometer which adjusts the amount of positive bias potential applied to the screen', grids' of keying tubes V2 and V3. The screen grid` electrodes of tubes V2 and V3 are connested by resistances Rl' and RS respectively, to the negative terminal of?. a source El, the positive terminal. ot which. isgrounded. Thescreen grids of. .tubes` V2. and. V3l are also connected by radio frequency bypassing condensers C to the cath-V odes..

The crystal controlled oscillators in units l0 andllare coupledby. coupling, condensers, l5 and l5 to theV control. grids of the tubes V2.V and' V3 While low. radiov frequency potential points on both of the. crystal oscillators in units lll and 2B are connected to-ground and to the cathodes of tubes V2; and V3by a resistance R3 shunted by a coupling and bypassing condenser O. Direct current grid circuits for V2 and V3 are completed through the resistances Rill and Rl l.

The signal source at A which may be unidirectional currents or potentials of opposite or reversible polarity representing mark and space respectivelyare supplied to the winding of a relay BLE having. an armature 2i cooperating with two contacts 22 and 23' to connect the positive terminal of: the direct current source to the screen grid of tubeVZ or tothe screen grid of tube V3 by way of resistance 35 and one or the other of contacts 22' 0123, depending upon whether' niark or space signal* is being supplied from source A.

' 'Phefrequencies of the oscillations developed in- I0'and 2U are usually made to have the desired prescribed frequencyV difference ofabout 100 or moreA cycles depending upon the needs o the particular system at hand'. The relay RE is opera-ted" from theY signal source A. The relay contacts 2l, 22 andY 23 alternately change the screen grid bias potentials of the switching and amplitying tubesVZ and V3 from a negative to a positive; value. When contact 22 or 23 is closed the screenY grid of tube V2V or V3 depending upon which contact is closed is connected to the positive terminal of the source. This alternately switches excitation from crystal units ill and 2B on the oscillator stage tube Vl because the oscillations are amplified or repeated through one of the tubes V2'y and V3 andA supplied to the control grid circuit ofv tube Vl. In the mid position of the relays RE negative'- bias is applied tov both switching tubes V8v andl V3 from source B through the resistors El' and R8. In this condition no excitation fromeither crystal oscillator le or 2E is; allowed to pass through the amplifying and keyingstage. During this time oscillator Vi is operating on its frequency determining circuit CEE Ll and N, unless. otherwise controlled or entrained.. In-v this embodiment a mid-frequency oscilla-tor` oliy substantially fixed frequency at supplies; oscillations by coupling condenser i9 to a-.-po iutY cin-.the potentiometer resistance 2l connected to the grid of tube VI to entrain this oscillator to operate at the mid-frequency or no signal frequency, when relay RE is in its midposition.

Now assume that the signal from A closes the relay contacts 2l and 23. Positive bias is applied to the screen grid of amplifying and switching tube V2, which repeats and possibly amplies the excitation from crystal oscillator lll, iinpresses it on the control grid of oscillator tube Vl through potentiometer R6. Sincethe oscillator Vl is as described above, somewhat flexible in operation, it has its frequency pulled so that it synchronizes with the frequency of crystal oscillator Ill. This new frequency is now transmitted and may represent space or mark. When the signal from source A reverses the relay RE changes and closes contacts 2| and 22. During the changeover period there is a short interval of time when negative bias is applied to both tubes V2 and V3, and the excitation from both crystal oscillators lll and 20 is cut olf. Then the oscillator VI swings smoothly back towards its own frequency of operation, that is, mid-frequency. When the relay contacts 2l and 22 are closed positive bias is applied to the switching tube V3 which repeats and possibly amplifies the excitation from the crystal oscillator in 2B, impressing it on the control grid of oscillator tube VI to thereby pull its oscillations into synchronism with the oscillations of the crystal oscillator in 20.

A feature of my present invention is that the oscillator VI and the oscillations generated thereby are not synchronized instantly with the oscillations produced by units I and 2U. It may take several cycles of oscillations from l0 and 2l! before the oscillations developed in tube Vl are locked in synchronism. By adjusting the potentiometer 21 the amount of control excitation potentials from the crystal oscillators in H) and to the controlled oscillator Vl may be set to give the desired pull-in or synchronizing time. The effect is a rounding olf and slope in the signal characters. Reducing the excitation voltage will increase the rounding and slope of the signal characters. Too much reduction will cause loss of signal or erratic operation. If a large amount of excitation is impressed on oscillator tube VI from the keying tubes, the signal characters will be more square shaped at the corners and more side frequencies will be generated. During the shifts in frequency the output of oscillator Vl remains substantially constant in amplitude. l

Although, as pointed out above, the shift between mark and space frequencies is smoothly carried out, it must be understood that the action is positive under all circumstances provided, of course, sufficient control is applied at 21 to synchronize the oscillator Vl with one or the other of the sources in il) and 20. The action is positive because when tube V3 is keyed on, that is, has its screen grid connected to the positive source of direct current by contacts 2l and 22, tube V2 is shut off due to the fact that its screen grid is connected by resistance R1 to the negative terminal of source B.

In the amplifying and switching circuit, R9 is a potentiometer and a point thereon is connected by a resistance I1 to the positive terminal of the main direct current source.l This permits me to raise the cathodes of tubes V2 and V3 positive with respect to the grids which are connected to ground by resistances Rl and RI l. This provides a means of maintaining a fixed bias on tubes 6 V2 Vvand V3 which is necessary to obtain definite cut-off of the current in these tubes. f

Potentiometer resistance 21 is a means for adjusting the amplitude of the control voltages on the grid of Vl, while potentiometer resistance R9 l and resistance 35 are means for adjusting vthe bias on tubes V2, V3.

In operation oscillations from lll of substantially xed frequency and of a frequency which is desired as a mean or non-signal frequency are supplied to a point on 21 below the point on 21 at which the anodes of tubes V2 and V3 are coupled. The oscillator VI generates oscillations of a frequency determined by CT and Ll which is preferably the same as the frequency of theI oscillations developed in 10. In any-event, Vl is controlled to operate at the desired mean frequency. Now as the switching tubes V2l and V3 are controlled by the reversed current signals from source A through relay RE the tube V2 or V3 (one thereof being cut off as described above) amplies oscillations from the crystal oscillator l0 or 20 and supplies them by coupling condenser B5 to the point on 21 to control or entrain the oscillations developed in VI on marking or spac-l lator 10 controls the frequency at which the generator Vl operates.

V2 or V3 takes over control when mark or space is applied, The excitation of the grid of Vl by the output of V2 or V3 therefore is made stronger than the excitation of the grid of VI by oscillations from oscillator 10. The feedback through N is such that oscillations are generated continuously in the tube Vl of a frequency determined by CT and LI. The control voltages applied at 21 act on the electron stream to entrain the generated oscillations.

The condensers C connecting the keyed screen grid electrodes of tubes V2 and V3 to the cathodes of the respective tubes in Figs. 2 and 4 and the condenser CI connecting the screen grid of the tube V2 to the cathode in Fig. 3 are bypass condensers for voltages of radio frequency such as generated in lsources l0 and 20, but these condensers are not to be so large as to distort audio frequency characteristics, otherwise the keying impulses of higher frequency might be greatly reduced in amplitude. However, these condensers may be made as large as .1 mf. or .3 mf. for the purpose of rounding off the signal characters if very high keying speeds are not used. In the yprior figures and in Fig. 3, to be described hereinafter, the letter O is applied to condensers such as between Ll and the cathode in Fig. 2, which serve primarily as bypassing condensers but may also serve as blocking condensers. These may be of relatively large capacity such as .01 to .1 without doing any harm, and depending on the operating frequency.

As stated above, the arrangement of Fig. 3 is a modication of the arrangement of Fig. 2. Note in general, that if the tube V3 and source 20 of Fig. 2 were omitted the result would be an embodiment substantially as illustrated in Fig. 3.

In Fig. 3 the crystaloscillator 1 is operating at say the spacing frequency, while the oscillator l0' operates at the marking frequency. The controlled oscillator Vl which with respect to the 7l., oscllationfgeneratoris asin; Fig. I2; isfsetrto -oper ate at a frequency slightly above-fthe.:spacingffre.- quency at which source i I operates. Theosource ltzhasrone l1termnal:connected .to .groundifand'the othersterminal coupledfby condenser I 9; and po.- tcntiomcter resista-nce `P2 tofground- Theanode of tube V2 is coupled by coupling and blocking condenser O5 through a potentiorneterfresistance Pl. to-:g-round. lThe cathode of tube Vl is connected ftofground 'by a cathode resistancezshuntedi: y; agradio frequency bypassing condenser-Q. condenser is of the same generaisizeas the condenser O, described -herei-nbefore. 'Theyontrol-grid oftubeVl Lis coupled .by La resistance and atta-p' P31 to a point Von potentiometer-P2fandby a resistance and a .tap P4 to a point on .the potentiorneter Pi. Y

.""lheyrelay RE has its actuating wi dinglseonnected-to a sourceof reversibilecurrent'Aas-in EigEZ, ibut-now has its armature 2l biased by a spring-.SFandconnectedby afresistance 15 to the direct currentsource. The screening. 616C- trode oftube V2: -isconnected by resistancefi to the; negative endfofy source `B as in Fig. 2, or by contact to tbe-.armature l2| and thence through resistance 'I5 to the positive terminalof .the :direct current-source. -The control `grid of tube V2 is biasedfbygesource Bi through resistance Rl 0.

-The-potentiorneters PI `and P2 and the points themen-andthe intensity Aof the source l and the gainvthrough switchingtube V2 etc., `are so arlaged' thatthe oscillator V! is lightly controlled byrrnndyllchr-onized with oscillationsfrom the source; that is, thespace frequency. When the eoliliiatslliand lof relay RE are. closed by pulses g' A, thenegative ,bias on the tube V2 from soupire-B. is overcome by positive potential supeli-ed through yresistance 'i5 from themain source andthe tube Vzamplifles the oscillatonsapplied toptheigrid from source 4lll and feeds to the control'fgridof tube 'Vl by way of potentiometer Pi andtap P4 .the excitation voltage of the markingf` frequency. This marking, excitation. is made considerably stronger than the spacing excitation soonest `tllecontrolled'oscillatortube Vl Will now bewcontrollcd by :and synchronized with the markfirequency of source IB' rather than the spacinglfrequency source 7. When the keyed impuise :outof A falls the field ofthe windingin Rpabeoomes vWeakenthe springgSP moves the armaturen from contact 2i thereby removing theiposirtive potential from the screen grid of V2 whiohgisfcut off and the source l again takes control-*to -operate Vi! `at the spacing frequency,

tris-.obvious that .the embodiments described hereinbefore -.may be modified in many Ways and many;.other:,circuit arrangements may be made to,.,=c2:1ry out the invention.; For instance, in Fig; i12: thesource'i may be omitted if one of thefgkeyingwtubes; say V3, is arranged to not key butV instead to pass continuously an adjustable smail- .amount of excitation voltage from the crystal. oscillator in 2B to the grid of the controlledor synchronized oscillator in the tube VIffjthis' frequency out of 20 could be used as the spacing frequency. Then when tube VV2 .is keyed tofamplify oscillations from the source IB a larger amount of marking excitation fromv this source l@ is-applied to the control grid of tube VI to take con-trol and:synchronize the oscillator VI with the markingffrequency out of source H1. When the relaying contacts 2'! and 23 open then oscillator'ffi/I vdrifts back' to 'thegspace frequency -of operation, Vthat is,I the frequency generated returns to syn'ch-ronism with the l"frequency-.of .oper

8 ation .fof-thesource in 2G. The arrangement of. Eiga may-be so operated Againreferrngto Fis. 3, if the point Pit-on potentiometer P2 is set at the bottom so that no excitation is .obtained from source '1, thenthe natural oscillations of oscillator VI will bethe space frequency and when the mark frequency from source l0 isswitched onV through tube V2 the oscillations of'tube-VI will be synchronized with it. In. this Way, by adjusting oscillator VI thespacing frequency may .be set about or below the inarkrfrequency as desired. Also the natural oscillations of ycircuit VI maybe used as the mark frequency and the crystal controlled frequency .may be used asthe spacing frequency.

IThe embodiment illustrated in Fig. 4 is a modication of the karrangementof Fig. 2. In Fig. 4,y thelcrystal oscillator 10 of Fig. 2 and its coupling tothe oscillation generator including tube VI is omitted. The anodesof the keyingtubes .V2'and V3 whichin other respects are` similar-to .the arrangementin Fig. 2,v are coupled by the coupling condenser 05 to ground through the poten tiorneter resistance Rl and .a point on this re-f. sistance is coupled through a coupling .condenser 'l5 to lthe second grid in the tube Vi. y.

As to the oscillation generator the tank .Circuit CT, Li is .coupled4 between the control grid and the anode. The-control grid is biased by a resistance 19 and the strength .of oscillations {or regeneration ,is vestablished as in the prior figures by adjustable regeneration condenser N. The remaining portions ,of the circuit andthe operation thereof are 1believed evident from thel prior description and fromthe drawings. rfhe ernbodiment of Fig. 4 was found to operate very favorably for the purpose designated, that is, generation, control and transmission of frequency shifted currents. f

Thecontrolled generator including tube Vi, vof all of the embodiments may include frequency stabilizing means if desired. The controlled gena erator Yof sayfFig. .4 is then -arranged as shown in Fig. `5. Grid `leak resistor Scand plate supply resistor 82 are in effect each connected across one-half of the tank condenser CT. These resistors also shunt the grid and anode tube resistance. and 82 are made -to have approxi.-l mately equal resistance value, which is small. relative `to the tube resistances. Those used in one embodiment were 10,000 ohms each. A lower resistor value would Work better but the oscillator would also give less output. Coils of the value L2/2 were also connected at the locations marked byX between the endsy of the tank circuitCT, L2 and the condensers S5. This. gave a small amount of improvement.

Referring again to Fig. 5, it may be statedthat the condensersrmarked O5, l5, B and 81 are low impedance .bypass condensers. It is known tothe art that if a resistance is connected across a tuned circuit a variation of the total resistance, such as that which Would be caused by temperature change, `does not change the resonant fre quency. Also sincev 80 andiare small comp aredwithV the tube resistances a variation of tube resistance, such as that caused by voltage changes (tube current changes) and .the like have 'very -little effect on the-'oscillator frequency. The-capacitychanges also have small effect on the operating frequency, especially .when the L2/2 coils are vused in the connection irl- .dica-.ted by X. It should be-noted that these frequency stabilizing means can .be applied .to any ofthe circuits .ofthis invention. K

amaca It was also found that, by making the capacity of condenser CT relatively larger and by making the inductanceof coil Ll or L2 relatively smaller than that generally used, the changes of capacity and resistance of tubeVI have a much less effect on the frequency of the oscillator circuit. Since the frequency deviation used for frequency shift keying is relatively small, stiffening the tank circuit by the use of a larger condenser and a smaller coil `caused no diiiiculties in modulation, in fact improving the fly-wheel effect of the oscillator. Of course CT and Ll or L2 should be made of materials that will cause minimum changes of capacity and inductance with changes of temperature or humidity.

What is claimed is:

1. In a signalling system a source of oscillations of controllable frequency, a source of oscillations of a first and substantially fixed frequency coupled thereto, a source of oscillations of a second and substantially fixed frequency, a source of oscillations of a third and substantially fixed frequency, and a switching device controlled in accordance with signals for alternately coupling said second and third sources of substantially fixed frequency to said source of controllable frequency for controlling the frequency of the oscillations generated in said source of controllable frequency in accordance with signals.

2. In a signalling system, a tube generator of oscillations of controllable frequency, a source of oscillations of substantially fixed frequency coupled to said generator for entraining operation of the same, a second source of oscillations of a second and substantially fixed frequency, a third source of oscillations of a third and substantially fixed frequency, and a pair of switching tubes controlled in accordance with signals for alternately coupling said second and third sources of substantially xed frequency to said source of controllable frequency for entraining the same with one or the other of the second or third substantially fixed frequency sources.

3. In a signalling system, a controllable oscillator operating at a first frequency, three control oscillators operating respectively, one substantially at said first frequency, another at a frequency above, and the third at a frequency below said first frequency, and means controlled by signals for coupling at least one of the control oscillators to the controllable oscillator to entrain the same to operate at the frequency of operation of the control oscillator coupled thereto,

4. In a signalling system, a source of oscillations of controllable frequency, a source of oscillations of a first and substantially xed frequency, a source of oscillations of a second and substantially fixed frequency, a source of oscillations of a third and substantially fixed frequency coupled to said first source for entraining the same for operation at its frequency, and a switching device controlled in accordance with signals for alternately connecting one or the other of said iirst two sources of oscillations of substantially fixed frequency to said source of controllable frequency for controlling the frequency of the oscillations generated in said source of controllable frequency in accordance with signals.

5. The method of signalling by frequency shift keying which includes these steps, producing oscillatory energy the frequency of which is readily variable, producing oscillations of a first and substantially iixed frequency, entraining said iirst mentioned oscillations by said second mentioned oscillations, generating oscillations of a second and substantially fixedy frequency higherthan the frequency of said first oscillations of substantially iixed frequency, v generating oscillationsof a` third and substantially fixed frequency lower `than the frequency of thefirst vmentioned oscillations4 of fixed frequency, entraining said oscillations of readily variable frequencyalternately in accordance with said oscillations of second and third fixed frequency, generating signalling currents the characteristic of which alternates from a condition indicating mark to a condition indicating space and vice versa, and controlling the entrain ing of said oscillations of readily variable frequency by said oscillations of said second and third substantially fixed frequency in accordance with changes in the character of said signals.

6. In a frequency shift signalling system, a regenerative tube generator of controllable frequency, a source of oscillations of a rst and substantially fixed frequency coupled with said generator for entraining the same lightly to operate at a iirst frequency, a second source of oscillations of substantially xed frequency higher than said first frequency, a third source of oscillations of substantially fixed frequency lower than said first frequency, two electron coupling tubes each having input electrodes coupled respectively to said second and third sources of oscillations of substantially fixed frequency and having output electrodes coupled to said regenerative tube generator to alternatively entrain the same and control the frequency of operation thereof, a source of signals comprising energy keyed in amplitude between two values representing respectively mark and space signalling condition, connections between an electrode of each of said coupling tubes and a negative and a positive source of direct current potential, a switch in at least one of said connections to each of said electrodes, a relay cooperating with said switch and controlled by said signals for applying a negative potential to an electrode in one of said coupling tubes and a positive potential to an electrode in the other of said coupling tubes and vice versa as said signal energy is keyed between mark and space values or Vice Versa, and an output circuitcoupled to said regenerative tube generator.

7. In a frequency shift signalling system, a regenerative generator of controllable frequency, said generator comprising an electron discharge device having an anode, a cathode and two gridlike electrodes, a circuit tuned substantially to the desired operating frequency coupling the anode, cathode and one of said grid-like electrodes in a regenerative oscillation. generating circuit, resistive means coupling the other of said grid-like electrodes of said tube to the cathode thereof, a first source of oscillations of substantially fixed frequency coupled to said resistive means for entraining said regenerative generator to operate at the frequency thereof, a second source of oscillations of substantially fixed frequency different from the frequency of said last mentioned source of oscillations, an electron discharge coupling tube having input electrodes coupled to said last named source and having output electrodes coupled to the resistive means connected between said other grid-like electrode and the cathode of said first device, a source of signals which varies in magnitude between two values, one of which represents mark condition and the other of which represents space condition, a control electrode in said coupling tube, direct current supply circuits therefor including potential sources of different value, and switching means 

